The invention resides in a rider of the type witch translates in bilateral directions along a rotating screw to move a load which is attached to it. The rider has a centralizing or radially stabilizing bushing that reduces vibration. The centralizing bushing maintains contact, alignment, and general stability between the bushing and a shaft and reduces noise and fluctuations between the bushing and the shaft when moving relative to each other. The bushing is constructed so as to maintain the relationship between the rider and shaft while ignoring temperature changes.
It is important that a rider, such as a nut or hub, which mounts a load, particularly one which is cantilevered, and which translate along a shaft or a rotating screw be free from radial play or transverse motion of the shaft axis. A typical mechanism is shown in our U.S. Pat. Nos. 4,566,345 and 5,601,372.
If, for example, the load were a laser device which aims at a target at a substantial distance from the laser, looseness of the mounting nut relative to the screw, of even a few ten-thousands of an inch, can result in the laser beam missing a target by feet, even though the angle of error is very small.
Conventionally, such nuts and screws have both been made of steel or other like metals. There are a number of disadvantages of running steel-on-steel. Periodic or continuous lubrication is required, otherwise the metal will be subject to spalling, seizing, or galling. However, in many instances, lubrication must be limited, if not completely prohibited, due to the environment in which the mechanism is being operated. The lubrication between a nut and a screw could inadvertently become transferred to items manufactured in a manufacturing process, soiling the products and making them potentially unsaleable.
Another problem is noise. Often, metal running on metal is noisy when there is a degree of looseness, regardless of lubrication, because of harmonic and other vibrations induced in the metal.
Attempts have been made to overcome the lubrication and noise problems by constructing the bearing of one metal and the shaft of another, the bearing usually being made of a softer metal than the shaft. This solution itself creates problems due to galvanic action and from temperature changes inducing inconsistent thermal expansion or contraction.
A circular bearing grows radially with heat, with both its inside and outside diameters expanding. Conversely, cold tends to reduce both the internal and external diameters. Similarly, the bearing and screw will expand axially when exposed to an increase in temperature and contract axially when exposed to a decrease in temperature. Consequently, a shaft or screw of one metal or a threaded rod rotating hi a circular bearing or threaded nut of another metal may either bind or become loose, in both the axial and radial directions, with temperature changes.
The amount of expansion or contraction of a metal due to heat or cold is a function of its coefficient of thermal expansion expressed in inches, per inch, per degree Fahrenheit (in/in/.degree. F.). If the bearing or nut has a higher coefficient of expansion than the shaft or screw, upon being subjected to heat, looseness resulting in play will be created between the screw and the nut resulting in noise and inaccurate operation. Conversely, if the nut and the screw or the bearing and shaft become colder during operation, as for example from an operation moving from indoors to outdoors, and if the coefficient of expansion of the nut is greater than the screw, the internal diameter can decrease and the parts could seize.
Other problems also exist in maintaining a bushing within a rider once inserted. As the rider translates along the shaft or rotating screw, the rotation of the shaft coupled with the longitudinal motion of the rider can cause the bushing to loosen within the rider and subsequently move along the axis of the shaft separately from the rider. Also, when exposed to changes in temperature, the bushing can expand axially and radially. While the radial expansion of the bushing would be fully constrained by the rider, the axial expansion would be constrained by the rider only along one bushing surface. Thus, the bushing would be free to expand outwardly of the rider. In either situation, once the bushing moves partially out of the rider, it becomes ineffective in removing vibrations between the rider and the shaft. Similarly, once the bushing has migrated from the rider, it can no longer maintain shaft stability and proper shaft alignment.